Electronically-switched DC motors, as well as stepper motors (or more generally brushless motors), are used in numerous control and regulation applications, and also in drive systems of mass memory devices such as hard disks, floppy disks, optical disks, CD-ROMs, tape streamers and the like. These motors are typically poly-phase motors having windings connected in a “star” or polygonal configuration. Hereinafter reference will be made to a three-phase electric motor with star-connected windings, though the same observations hold, with minor changes, for an electric motor with delta-connected windings and for a generic poly-phase motor.
Brushless motors may be driven using a switching controller of the type shown in FIG. 1, commonly referred to as a “smooth driver”, and illustrated in detail in U.S. Pat. Pub. No. 2011/0156622, the disclosure of which is herein incorporated herein in its entirety by reference. The output stage is represented by a poly-phase full-wave bridge circuit which, in the case of a three-phase motor, may employ six bipolar (BJT) or field effect (MOS) power transistors.
Typically, a brushless motor is driven by properly powering the motor phases synchronously with the instantaneous position of the motor. This may be done by sequentially exciting two of the motor's windings with positive and negative voltages respectively, leaving a third winding in a high impedance state (tri-state). When driving a “sensorless” brushless motor, the non-excited phase winding is exploited for detecting the rotor's position. Excitation voltages or currents are applied to the phase windings of the motor and, instead of having a certain pre-established constant level during each switching phase, a certain pre-established (non-constant) drive voltage or current profile, respectively, is digitized and stored in a nonvolatile static memory device, e.g., in an EPROM or EEPROM memory. This technique is described in European Patents EP 800262, EP 800263, EP 809349 and from the U.S. Pat. No. 6,137,253, the disclosures of which are hereby incorporate herein in their entireties by reference, and thus will not be discussed further herein.
To properly energize the windings, the position of the rotor of the motor is determined with a circuit (which in FIG. 1 is provided in block 5) for sensing the back electromotive force (BEMF) induced in a tri-stated winding. For a star-connected three-phase motor, such a sensing circuit may be as depicted in FIG. 2. Each winding of the motor is schematically represented as a series connection of a resistor R (representing the series resistance of the winding), an inductor L (representing the inductance of the winding), and an oscillating voltage generator BemfA (BemfB or BemfC), which represents the back electromotive force induced in the winding by the motion of the rotor of the motor. In FIG. 2, Va, Vb and Vc are the driving voltages applied to the phase terminals of the motor, Vct is the center-star voltage, and Ia, Ib and Ic are the phase currents flowing throughout the windings of the motor. In the ensuing description, the expression “phase voltage” will designate the difference between the driving voltage of any winding (Va, Vb, Vc) and the center-star voltage (Vct), and the same labels in the various figures indicate the same components.
A first known method for sensing the back electromotive force BEMFA includes generating a time window of a short duration during which the output node of one of the half-bridge stages that drive the windings of the motor is tri-stated, as schematically illustrated by the time graph of FIG. 3. This is represented by the following equation:VA−VCT=BEMFA+R·IA+L·∂IA/∂t Thus, when the winding A is tri-stated, the phase current IA flowing therethrough and its time derivative are null, and the back electromotive force BEMFA induced thereon equals the phase (star) voltage VA−VCT. This method includes the steps of:                tri-stating at least a winding of the motor; enabling a logic gate asserting a zero-cross event of the back electromotive force BEMF sensed by a sensing circuit after a certain time from the instant in which the winding is tri-stated; and        resuming from the high impedance state after the assertion of a zero-cross event.        
This technique produces discontinuities in motor driving torque. It also generates a certain amount of acoustic noise, and induces distortions in the other motor phases that are not temporarily tri-stated, as shown in the exemplary time graph of FIG. 4.
According to another technique, the back electromotive force BEMFA is reconstructed in an analog fashion with a circuit for reconstructing the phase voltage Vpha and the phase current Ia, e.g., using the circuits shown in FIGS. 5 and 6. In these prior circuits, the phase current la is sensed with a sense resistor Rs, connected in series to a generic winding A, and the sense voltage Ia*Rs and the phase voltage Va−Vct are amplified by operational amplifiers coupled to the winding through a network of input and feedback resistors R1, . . . , R8. In the circuit of FIG. 5, the reconstructed phase current Ia is processed by a circuit GAIN AND DIFFERENTIAL BLOCK for generating a signal representing the voltage drop R·IA+L·∂I*A/∂t, to be subtracted from the reconstructed phase voltage Vpha for generating a replica of the back-electromotive force BemfA of the winding A. Eventually, as in the circuit of FIG. 6, the reconstructed voltage and current Vpha and Ia are converted in digital form using analog-to-digital converters (ADC) and the digital replicas thereof are processed by a calculation circuit, such as the FPGA (Field Programmable Gate Array) shown in the figures, that generates a digital replica of the voltage BEMFA.
This technique requires additional analog components, i.e., at least a sense resistor Rs in series to the winding and operational amplifiers coupled thereto through a network of input and feedback resistors R1, . . . , R8, for generating a replica Vpha of the phase voltage Va−Vct and/or a replica of the phase current Ia.